1. Field of the Invention
The present invention generally relates to methods for electroplating a metal onto a plastic substrate. More particularly, this invention relates to a method for electroplating plastics having a high impact resistance wherein the plastic substrate is a polycarbonate which has been modified with up to about 50 percent of an acrylonitrile-butadiene-styrene polymer.
2. Description of the Prior Art
Motor vehicles often include substantial amounts of chrome plated trim elements which provide both decorative and functional purposes. Such trim elements include bumpers, body side moldings, lower body claddings, wheel covers and grilles as well as other components. The overall appearance of the vehicle is significantly enhanced by these highly reflective and glistening chrome surfaces. However, these trim elements also serve a functional purpose in that they help to absorb the impact when the vehicle is involved in a collision or when the vehicle contacts flying gravel, road debris, roadway abutments or the like.
Traditionally these trim elements have been manufactured from materials which are capable of absorbing such an impact, examples being chrome plated steel, anodized aluminum, and stainless steel. These traditional trim elements not only add significantly to the cost of the vehicle, but substantially increase the vehicle weight thereby decreasing the fuel economy rating for the vehicle.
Therefore, there has been great efforts by the automotive industry toward developing a cost effective, lightweight alternative to these chrome plated metals. Plateable plastics are such a desirable alternative, because they not only reduce the vehicle weight and thereby correspondingly increase the vehicle fuel economy, but plateable plastics also allow for parts consolidation within the automobile. Plastics have much greater design flexibility than metals. Plastics may be easily molded into a limitless variety of complex and contoured configurations which cannot be achieved with conventional metal stamping and forming operations. In addition, when these parts are formed from plastic materials, a significant cost savings is realized over comparable parts formed from metal.
A wide variety of plated plastics are known. For example, unmodified acrylonitrile-butadiene-styrene (ABS) has been plated to provide decorative articles such as headlamp surrounds, and plumbing and marine hardware. Unmodified polycarbonate (PC) has been utilized as the substrate for plated motor vehicle door handles. In addition, several other plastics have been successfully plated for various decorative purposes. However, these plastics, even though plateable for decorative purposes, do not provide a satisfactory substrate if the finished article must be capable of sustaining significant impact. Accordingly, the use of these materials within an automobile is limited. These plated plastics are characterized by a tendency to fail at low energy levels of impact. Regardless of the substrate utilized, upon impact the chrome plating typically delaminates from the substrate. In addition, as an extreme example, the unmodified ABS may even shatter upon impact.
Thus, for a plastic to be suitable as a substrate in a plated article within an automobile, it must generally have sufficient impact resistance. Further, its impact resistance must be good over a wide range of temperatures. Attempts to develop an impact-resistant plastic have resulted in the formulation of many blends, including acrylonitrile-butadiene-styrene polycarbonates which will be referred to throughout as ABS-modified polycarbonate polymers. When the ABS material is compounded with the polycarbonate, the result is a material characterized by a good balance of heat resistance and impact properties over a wide range of temperatures, all at an intermediate cost.
However, until this invention, commercial efforts to plate an ABS-modified polycarbonate substrate have been unsuccessful. To the extent that the metal plating can even be deposited onto the ABS-modified polycarbonate material, there is little or no adhesion to the substrate. Therefore, upon impact, the metal plate chips, cracks, fractures and delaminates.
Methods have been taught by the art to improve adhesion between the metal plate and a plastic substrate. In particular, U.S. Pat. No. 4,125,649 to Donovan et al., entitled "Pre-etch Conditioning of Polysulfone and other Polymers for Electroless Plating" teaches one method for conditioning the surface of various polymer substrates to enhance adhesion of subsequently deposited metals to that surface, including unmodified ABS material and unmodified polycarbonate material. Donovan et al. successfully condition the various polymer substrates using a halogenated alcohol, 1-3-dichloro-2-propanol, and then plate onto the substrates with satisfactory levels of adhesion resulting.
However, the time, temperature and concentration of the organic solvent conditioner were optimized by Donovan et al. to generally provide a high activity, very aggressive conditioner. With regard to the unmodified polycarbonate, it is necessary to micro-crack and roughen the unmodified polycarbonate so as to provide a suitable plating surface. It has been determined that conditioning an ABS-modified polycarbonate substrate with the type of highly aggressive conditioning treatment taught by the method of Donovan et al., would be detrimental and undesirable because the conditioner overly reacts with the ABS-modified polycarbonate substrate, thereby damaging the surface finish of this type of substrate and making it unacceptable for subsequent plating.
Donovan et al. are thorough in their exhaustive list of polymer materials which may be conditioned using their method to enhance adhesion of the metal plate on a plastic substrate, yet the high impact resistance ABS-modified polycarbonate material is noticeably absent from their teachings. This is typical of the art, since there has been no satisfactory method, as of yet, to effectively plate this high impact resistant material so as to result in a high level of adhesion between the metal plate and the plastic to prevent delamination upon impact.
It is to be noted that other impact-resistant polymeric blends, such as nylon-polycarbonate blends, unblended polyurethane, and butadiene-loaded ABS have been plated. However, these materials exhibited unsatisfactory results when plated because of their poor cosmetic appearance and failure to withstand impact without delamination. Therefore, these materials are also unsuitable for high impact plated plastic applications.
Accordingly, what is needed is a method for electroplating metal to a high impact plastic, particularly an ABS-modified polycarbonate substrate, wherein the resulting product is characterized by excellent adhesion between the metal plate and the ABS-modified polycarbonate substrate.